My favorite experience had to be in a UMP2 level (Forget name).
I was stalking a marine it was dark. I saw him as he ran down the corridor, so I leaped to the wall, he turned when he heard me, but saw nothing. I followed him down the corridor, hiding every time I made noise. He saw me once, started to fire, blasted me with grenade. I hid behind a box, almost no health, he though dead of me. I kept following him, him coming to a large pit-like room. There was an exo in it. I crawled on the celling, him getting in the exo. As he was about to leave, I leaped down and he heard me. He turned and saw me, started to fire again. He randomly fired, and I jumped to the celling again, him loosing sight of me. After he used up all his minigun, he backed against a wall. I crawled right above him, LITERALLY. RIGHT ABOVE HIM. I was ready to head-bite, but I could tell he was scared out of his mind. I knew this because he typed something to this extent to his marine buddy.
"Get down here! Come help me! I can't find the thing! It disappeared! It's just GONE!"
After a moment or two, it opened fire with missiles, flame, plasma, everything. I head-bit him right there. Never knew what hit him.